In modern day aircraft, it is desirable to reduce the loads applied during flight to the aircraft structure, and particularly to the aircraft wings, so as to result in increased energy efficiency and increased payload or range capability of the aircraft. Since the wings of an aircraft are designed to be capable of withstanding certain maximum wing loads, with a predetermined load distribution in the spanwise and chordwise directions of the wing, it will be recognized that a redistribution or other alleviation of wing loads encountered during flight can also result in significant reductions in the design requirements for certain of the wing components, and therefore in the overall structural weight of the wings, therefore further increasing the energy efficiency and range or payload capability of the aircraft.
A number of wing load alleviation systems have been proposed which use the outboard ailerons of the aircraft for redistributing wing loads encountered during flight. As is well known, ailerons, both outboard and inboard, typically are used to effect roll control of the aircraft, in which case the ailerons are deflected differentially, that is, the ailerons on one wing are deflected upwardly to reduce lift on the associated wing, while the ailerons on the other wing are deflected downwardly to increase lift on the associated wing. In the proposed wing load alleviation systems, however, both outboard ailerons are deflected symmetrically upon detection of an excess load condition, typically occurring as a result of a pilot-induced aircraft maneuver or an air gust. Usually, each aileron is deflected upwardly so as to reduce lift on the outboard portion of its associated wing, with the result that the center of lift of each wing is moved inboard to therefore effect an inboard shift in the wing load distribution.
While the use of such wing load alleviation systems theoretically allows a reduction in the weight of the wings due to the decreased design requirements for the outboard portion of the wings, such a reduction has not been achieved in practice, for the deflection of the outboard ailerons at the speeds normally encountered in steady-state flight also induces significantly increased torsional loads on each wing, that is, the aerodynamic forces acting on each deflected aileron causes its associated wing to twist about its spanwise axis. In order to accommodate these significantly increased torsional loads, the design requirements for each wing must be increased so as to result in little or no overall reduction in the weight of the wings. Also, flexing of the wing upon the application of such torsional loads will result in a change in the effective angle of attack of the wing. When a predetermined airspeed is reached, typically less than the maximum certified airspeed of the aircraft, the phenomenon of aileron reversal occurs in which the decrease in lift caused by aileron deflection is balanced by the increase in lift caused by the change in the effective angle of attack. Upon aileron reversal, it will be seen that the aileron loses its effectiveness as a control surface and particularly in alleviation of wing loads.
It is therefore an object of this invention to provide an improved wing load alleviation system using outboard ailerons.
It is another object of this invention to provide such a system which achieves wing load alleviation without significantly increasing wing torsional loads encountered during flight.
It is yet another object of this invention to provide such a system which significantly decreases the probability of aileron reversal occurring airspeeds up to and including the maximum certified airspeed of the aircraft.
It is still another object of this invention to provide such a system which can be easily retrofitted to existing aircraft without the necessity of significant increase in the structural strength of the aircraft wings.